Same as with normal chin-up progression. Start at the top and lower yourself with one arm, if that doesnt work put one foot slightly on the ground to take away some weight.
That being said its a stupid thing to train for as one tends to ignore the weaker arm to get to the goal faster and it doesnt get you anywhere concerning overall functional strength.

Doing strict one-arms was always a goal of mine, finally achieved it a couple years back, now I can do 2 consecutively with each arm. Here's what got me there: first of all, just practice pull-ups with a lot of variations. Especially narrow grip though. Focus on narrow grip pull-ups because one-arms are done with your hand directly overhead (not out to the side like with regular pull-ups). Also, do pull-ups weighted. One-arms are a strength move, so put weight on when working your pull-ups and try to progress in weight like you would any lift. Finally, I definitely recommend assisted one-arms, but not the kind with a foot on the ground. Instead, get assistance from your other arm by putting it on a lower hold or something (somewhere were it can help, but where it's at an extreme disadvantage so it can't help much no matter how hard it tries).